to make it seem like the site is busier.
unfortunately for poker rooms you need at least 2 customers for a game, in a casino you only need one so they dont use shills.
In a new poker room you may not have many customers, so to guarantee that there is a game when players come to the site the house will employ a shill, or a prop.
They are usually wannabe pro players, they dont work directly for the house and have no privileges other than they get paid to play. usually a rakeback or hourly fee.
it tends to be the smaller/new sites that have to hire them as they will not have reached a critical self sustaining mass of traffic.
Shilling is very common and a totally acceptable way for a new poker room to get started.